After enabling copies between scatter gather lists (sgl_s), another
storage related operation is to compare two sgl_s. This new function
is modelled on NVMe's Compare command and the SCSI VERIFY(BYTCHK=1)
command. Like memcmp() this function returns false on the first
miscompare and stops comparing.

A helper function called sgl_compare_sgl_idx() is added. It takes an
additional parameter (miscompare_idx) which is a pointer. If that
pointer is non-NULL and a miscompare is detected (i.e. the function
returns false) then the byte index of the first miscompare is written
to *miscomapre_idx. Knowing the location of the first miscompare is
needed to implement the SCSI COMPARE AND WRITE command properly.

Reviewed-by: Bodo Stroesser <bostroesser@gmail.com>
Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
---
 include/linux/scatterlist.h |   8 +++
 lib/scatterlist.c           | 109 ++++++++++++++++++++++++++++++++++++
 2 files changed, 117 insertions(+)

diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 3f836a3246aa..71be65f9ebb5 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h

@@ -325,6 +325,14 @@ size_t sgl_copy_sgl(struct scatterlist *d_sgl, unsigned int d_nents, off_t d_ski
 		    struct scatterlist *s_sgl, unsigned int s_nents, off_t s_skip,
 		    size_t n_bytes);
 
+bool sgl_compare_sgl(struct scatterlist *x_sgl, unsigned int x_nents, off_t x_skip,
+		     struct scatterlist *y_sgl, unsigned int y_nents, off_t y_skip,
+		     size_t n_bytes);
+
+bool sgl_compare_sgl_idx(struct scatterlist *x_sgl, unsigned int x_nents, off_t x_skip,
+			 struct scatterlist *y_sgl, unsigned int y_nents, off_t y_skip,
+			 size_t n_bytes, size_t *miscompare_idx);